The recent advent of nonemissive electronic display technologies allows the manufacture of inexpensive displays having low power consumption requirements. These displays have uses in a variety of applications such as advertising, information dissemination, education, and entertainment. Because these displays are nonemissive, their effectiveness is limited to daytime use or to use in lit rooms. An illumination system that selectively illuminates the display medium of such a display system is therefore desired.
Traditional displays, such as, for example, liquid crystal displays, are typically are illuminated using backlighting. In backlit displays, light from a light source passes through the back of the display via a light transmissive element, such as a light pipe, and is perceived by a viewer. This type of illumination can be used with liquid crystal displays because light can effectively pass through the display medium. However, the utility of using backlighting with a nonemissive display, such as a microencapsulated electrophoretic display or rotating ball display, is limited because the display medium of such displays is effectively opaque, thus acting as a barrier to light. In addition, various elements used to increase the uniformity of light passing through the light transmissive element, such as diffusers, decrease the amount of light ultimately reaching the viewer.
Conversely, the design of a frontlighting system for illuminating reflective displays requires a trade-off between achieving uniform illumination of the display and minimizing undesirable reflection of light away from the display and towards the viewer, which lightens the dark state of the display and reduces its perceived contrast.
A common frontlighting scheme uses a plastic sheet that covers the display area. Light enters the front light sheet from the side, either from a tubular lamp or a microstructured light bar that diffuses light into the sheet from a point source lamp. The light then propagates across the sheet by total internal reflection, bouncing off the surfaces of the light sheet many times. Surface elements, e.g. micropatterns, such as serrated edges, pyramids, grooves, and others, are used to direct a fraction of the internally reflected light down through the display. Typically, these light-deflecting features make the light's angle of approach to the display steeper. The density of the deflecting features is usually graded across the display in order to compensate for the loss of intensity with distance from the source, thus achieving nearly uniform illumination of the display surface. These systems, however, generally suffer from backscattering because of the addition of two surfaces between the display medium and the viewer, i.e. the front and back surfaces of the front light sheet, as well as because of the micropatterns and other surface elements disposed on one of the surfaces of the front light sheet.